Prior art electric drives for bicycles can be divided into the following four basic types:
(1) Friction drive on the tire;
(2) Drives through the pedal shaft to the rear wheel;
(3) Direct drives to the rear wheel; and
(4) Wheel hub motors.
The cheapest and simplest type of electric drive for a bicycle is a friction drive on the front or rear tire. This method is so inefficient that it is almost impractical. However this type will probably continue to be built and sold, because they can be easily installed on an existing bicycle as a kit. U.S. Pat. No. 6,065,557 to von Keyserling, U.S. Pat. No. 5,316,101 to Gannon, and U.S. Pat. No. 3,961,678 to Hirano contain examples of this type of drive.
State of the art drives through the pedal shaft to the rear wheel are usually heavy, bulky gearboxes with electric motors attached and a pedal shaft protruding on each side. The advantage of this type of drive is that the rear wheel is driven through the normal pedal chain by the pedals and the motor, or by the pedals alone. Therefore, a normal multi-speed bicycle rear drive can be used to improve hill climbing ability and efficiency. There are a few versions that allow the motor to drive the rear wheel without turning the pedals, but they require additional mechanisms, which increase the cost.
A disadvantage of this type of drive is that the pedal shaft turns at about one third of the speed of the rear wheel in high gear; therefore, the rotational speed of the motor must be reduced about three times further when driving through the pedal shaft, than when driving the wheel directly. And then, as the power from the motor is transmitted on through the pedal shaft to the rear wheel it has to be sped up again, to about three times the pedal shaft speed. Both the additional reduction and the subsequent up-speed add to the friction losses and cause a significant loss in overall efficiency.
Electric bicycles must carry a large amount of battery weight to have an effective range, and for safety and maneuverability it is very important to keep that weight low and toward the center of the bicycle. Therefore, another disadvantage of driving through the pedal shaft is that the bulky transmission and motor combination around the pedal shaft causes the battery to be relegated to a higher position, away from the center of the bicycle. U.S. Pat. No. 6,230,586 to Chang, U.S. Pat. No. 6,131,683 to Wada, and U.S. Pat. No. 6,015,021 to Sonobe each disclose a different configuration of a drive through the pedal shaft to the rear wheel.
Direct drives to the rear wheel take many different forms, but one disadvantage common to all of them is that they require another drive chain and sprocket, or belt and pulley to the rear wheel, in addition to the customary pedal chain and sprocket. Also, in order to pedal the bicycle efficiently when the motor is not in use, another ratcheting device (commonly called a “freewheel”) is required between the extra sprocket or pulley and the rear wheel hub. U.S. Pat. Nos. 6,011,366 to Murakami, 5,937,964 to Mayer, and 5,433,284 to Chou are examples of direct drive to the rear wheel.
Wheel hub motors look similar to the normal bicycle hub having flanges with holes for spokes on each side and an axle through the center. However they are much larger in diameter, about six to ten inches, and much heavier, about ten to fifteen pounds. They are made for either front or rear wheel application, but when applied to the front wheel of a bicycle, they create a gyroscopic force that at high speed makes the bicycle hard to steer and dangerous in some conditions. When applied to either end of the bicycle, wheel hub motors increase the polar moment of inertia significantly in both the vertical and horizontal planes. It is well known by those skilled in the art of designing vehicles that this is a highly undesirable characteristic from a handling and safety point of view. When applied to the front of a bicycle with front suspension, the large increase in un-sprung weight renders the suspension practically ineffective, and the same is true at the rear. U.S. Pat. Nos. 6,286,616 to Kutter, 6,093,985 to Chen, and 6,015,021 to Tanaka teach different configurations of wheel hub motors.
At constant voltage, as the required torque increases, the speed and the efficiency of a direct current electric motor of the type used on electric bicycles decreases. Therefore, a bicycle with a single gear ratio electric drive is very inefficient when climbing a hill, because it must slow down to develop the required torque to overcome the hill. In moderately hilly terrain, this inefficiency can cut the range of the bicycle in half. The steeper the hill, the less efficient the motor becomes. On a long hill, this wasted energy usually heats up the motor enough to open the thermal protection switch and turn off the power before the bicycle reaches the top of the hill.
Common bicycle multi-speed drives, such as multi-speed hubs or rear derailleurs, drive the rear wheel through a freewheel device, so that the pedals do not turn while coasting. Therefore, any of the prior art bicycles that used this type of devices to increase the hill climbing ability of the motor do not have the ability to recharge the battery through the electrical generation capability of the motor while coasting downhill. In fact, there does not seem to be any evidence that there has ever been a bicycle with more than a single gear ratio electric drive that had the ability to drive the motor while coasting downhill. This ability is important to extending the range of the electric bicycle, because most electric motors have the ability to act as generators when connected to the correct circuitry.
Accordingly, the need exists for an electric powered bicycle that incorporates the following important features:
(1) A simple, inexpensive multi-speed rear wheel drive that can be driven efficiently by either the pedals or the motor independently or both in unison, without losing the ability to drive the motor as a generator for charging the batteries while pedaling or coasting downhill.
(2) A shifting device that can be used to shift the multi-speed drive to greater speed reductions as the bicycle begins to climb steeper hills. (When the hub shifts to larger reductions, the torque required from the motor to climb the hill is reduced and the efficiency increases.)
(3) A motor/drive unit and large battery container that can be fitted to an existing bicycle design in a position that is low and close to the center of the bicycle.